1. Field of the Invention
The present invention relates to high speed data communication (e.g., 1 G-bit/second or higher). In particular, the present invention relates to a bidirectional differential driver that maintains the same common-mode voltage for sending and receiving differential signals.
2. Discussion of the Related Art
Current-mode low-swing differential signals (LVDS) are often used in high speed data links to reduce electromagnetic interference (EMI), power consumption and noise interference. When a signal is transmitted over a significant distance between a transmitter and a receiver over copper wires, a technique—known as “AC coupling”—prevents a DC current from flowing between the receiver and transmitter as a result of a difference in ground or power supply voltages between the transmitter and the receiver on opposite sides of the transmission. AC coupling may be accomplished by inserting blocking capacitors into the communication link. This isolation technique allows two different common-mode voltages to exist at the transceivers at the ends of the communication channel. However, the different common-mode voltages may be limited or may cause errors, especially for bidirectional communication over a single communication link.
Full duplex is the preferred mode of bidirectional data communication. However, full-duplex bidirectional communication requires echo cancelling, and thus full-duplex mode is available only in low-speed data communication. At 1-Gbit/second or higher data rate, half-duplex mode is typically used. FIG. 1 shows schematically a bidirectional communication through link 100. At each end, a transceiver—consisting of both a transmitter and a receiver—performs the transmission and reception functions. For example, FIG. 1 shows transceivers 120 and 121. Transceiver 120 is shown to include transmitter 101 and receiver 102. Similarly, transceiver 121 is shown to include transmitter 103 and receiver 104. When transceiver 120 is in the transmitting mode, transceiver 121 is in the receiving mode. Within transceiver 120, when transmitter 101 is active (i.e., the transmission mode), receiver 102 is inactive. Conversely, in the receiving mode, transmitter 101 is inactive. In AC-coupled communication link 100, when transceiver 120 switches from a transmitting mode to a receiving mode, receiver 102's differential front end may experience an instantaneous large swing or surge in common-mode voltage, when the common-mode voltages are different between transmit and receive modes.
Such a voltage surge overlaps in time with the incoming signal and may overwhelm and blind receiver 102 until the surge condition settles down to within the receiver detection limits. When such a condition occurs, the receiver may lose its function until the common-mode voltage surge falls into the sensitivity range of the receiver. In a system where a differential termination resistor of 100 ohms and a blocking capacitor of 1 nF are used, the receiver may be blinded for a few hundred nanoseconds (ns). During this period, communication over the communication link is not possible.
FIG. 2 illustrates, in further detail, a general AC-coupled bidirectional communication system without common-mode voltage control. FIG. 2 shows the transmission systems of transceivers 200 and 250, which communicates over a communication link 270. As shown in FIG. 2, transceiver 200 includes transmitter 201 and receiver 202. Transceiver 250 is substantially identical to transceiver 200, except that transmitter 251 is disabled during receiving by grounding the input terminals of transmitter 251's differential driver circuit. In this configuration, transmitter 251 acts as a termination circuit during the receiving mode of transceiver 250.
FIG. 3 shows a waveform of output signal 212 from transceiver 200. As shown in FIG. 3, differential signal 212 is represented by the voltage difference between signal waveforms 212a and 212b. The common-mode voltage Vcom—TX in differential signal 212 is given by:
            V              com        ⁢        _        ⁢        TX              =                            V                      212            ⁢            a                          +                  V                      212            ⁢            b                              2        ,where V212a and V212b are the component voltages of differential signal 212. The common-mode voltage Vcom—TX in differential signal 212 is also given by:
            V              com        ⁢        _        ⁢        TX              =                            V                      dd            ⁢                                                  ⁢            1                          -                                            V                              212                ⁢                                                                  ⁢                a                                      -                          V                              212                ⁢                                                                  ⁢                b                                              2                    =                        V                      dd            ⁢                                                  ⁢            1                          -                              Δ            ⁢                                                  ⁢                          V              1                                2                      ,where ΔV1=V212a V212b is the voltage swing in the transmitter output during transmission, and Vdd1 is the supply voltage of transceiver 200.
FIG. 4 shows a waveform of input differential signal 214 at receiver 250 on the output side of the blocking capacitors. As shown in FIG. 4, differential signal 214 is represented by the voltage difference between signal waveforms 214a and 214b. Because the input terminals of differential pair 215a and 215b are both grounded, no DC current flows in the two 50-ohm termination resistors, so that no voltage drop across these termination resistors in transmitter 251, the voltage levels of signal waveforms 214a and 214b swing above and below supply voltage Vdd2 at termination circuit 251. The common-mode voltage Vcom—RX in differential signal 214 is given by:
            V              com        ⁢        _        ⁢        RX              =                            V                      214            ⁢                                                  ⁢            a                          +                  V                      214            ⁢                                                  ⁢            b                              2        ,where V214a and V214b are the component voltages of differential signal 214. The common-mode voltage Vcom—RX in this instance equals supply voltage Vdd2, and the voltage levels of differential signal 214 are
      V          dd      ⁢                          ⁢      2        +            Δ      ⁢                          ⁢              V        2              2  and
      V          dd      ⁢                          ⁢      2        -                    Δ        ⁢                                  ⁢                  V          2                    2        .  
As illustrated by this example, the difference in common-mode voltage between transmission mode and receiving mode can be 50% of the differential signal swing. In such circumstances, the receiver may require as much time as 5 times the relevant RC time constant (i.e., 500 ns) to settle to less than 1% of this difference. In addition to all the possible variations in the manufacturing process, the supply voltage and temperature changes during operations, additional factors, such as (i) tail current boost-up, and (ii) pre-emphasis techniques can influence the common-mode voltage of a transmitter. These techniques are common in high-speed data communication, where high frequency loss is compensated in a long or lossy link.
In a bidirectional link, maintaining or tracking the common-mode voltage in a transceiver for both transmitting and receiving modes is essential to minimize the blind period during mode switching.